Brian Rudman: Hear, hear for the acoustic engineer’s science

Could it be that the regional botanic gardens' notorious noise barrier wasn't guilty after all. And that the increased motorway noise that Manurewa neighbours claimed they heard after it was put up in January 2001 was all in the mind, not the ears?

That's certainly the conclusion of a paper in the latest New Zealand Acoustics Journal by Chris Day, of Marshall Day Acoustics.

In the weeks before the $300,000 wall's Environment Court-ordered destruction in March 2003, Mr Day took noise samples at three sites near where the main complaints had come from. Following the removal, he returned to repeat the readings. His just-published analysis reveals no difference in noise levels.

At residences 30m from the motorway in Lawrence Cres, noise averaged a head-aching 72.48 decibels with the barrier and a fractionally higher 73.3 decibels without it.

Further away in Frank Place (400m) the average of 43 decibels did not change, and in Patricia Place (650m), the average crept up from 44.8 decibels to 46.6 decibels with the barrier gone. However, in the latter two sites, there were variations of up to 17 decibels due to changes in traffic flow and weather.

Marshall Day Acoustics was the Auckland Regional Council's adviser on this ill-fated project. Mr Day was criticised by Environment Court judge Frederick McElrea for giving answers that "were not always reliable" during the hearing.

The judge declared: "We do not accept that the residents' evidence can be dismissed as subjective, anecdotal and unreliable when weighed up against 'so-called' scientific evidence. This is too simplistic." He added that "well-constructed surveys of residents can be a good indicator of noise increases".

In the two-year battle to have the 3.5m by 550m wall demolished, I sympathised with the residents' plight. Like the commissioners, I accepted the word of the dozens who claimed noise levels had gone up as a result of motorway sounds reflecting from the wall back across the motorway into their homes.

Marshall Day had argued at the time that this was not scientifically possible. Unfortunately for it and the ARC, this conviction that such reflections were not possible meant that only one "before" reading was taken before the wall was built. That was at a very noisy site adjacent to the motorway.

This meant that when complaints came from further afield, there was no "before" data to challenge the claims. The only way of scientifically assessing the wall's acoustic impact was to take it down and measure the difference. Which is what eventually happened, but by then the political battle had been lost.

No doubt some will see Mr Day's findings as self-serving. And of course they are, inasmuch as they're defending his company's reputation. But that doesn't mean they're incorrect, or biased.

I first met Chris 20 years ago during the building of the Aotea Centre and have often used him informally since to help with matters acoustical. I've never doubted his professionalism or expertise.

He invited me out on one of his late-night measuring sessions before the doomed wall came down. I remember his frustration, standing in dark suburban Manurewa, as we stared down towards the wall and heard the loudest noises coming not from that direction, but straight up a valley to the left from an unwalled stretch of motorway. We were outside the home of one of the main complainants.

His explanation now is the one he presented to the commissioners, who rejected it. It comes now with the scientific under-pinning then missing. He argues that local residents were upset at Manukau City giving consent for the wall without any chance to comment.

Up went a huge and ugly steel and polystyrene barrier, which blocked views into the botanic gardens.

For the two weeks following construction, north-easterly winds replaced the prevailing south-westerly breezes. Noise levels in north-easterly conditions are typically 15 decibels higher.

Residents linked the ugly new barrier with the increased noise, and the wall's fate was sealed.

For Mr Day, one lesson is that if there's the slightest risk of adverse community response, a project should be notified so residents get the chance to have their say.
The other is "that we do need to give weight to scientific evidence as well as to community response".